Less so. The way to calculate this is as follows:
- Imagine a map of political ideologies. You might pick the left->right political spectrum. You might pick a 2d map. You might choose certain issues to view as germane and certain issues to view as irrelevant. The actual geometry at play here is unimportant. Just pick a map where you feel comfortable mapping everybody in a representative way.
- Situate all of the candidates on the map.
- Situate yourself on the map.
- When you vote, you want to vote for the candidate who has the least overall distance from you on the map. For example, if you share 100% of your issues with Gary Johnson, he's at the same place as you on the map. The distance from you and other candidates depends on the issues you feel are important and the way you drew the map, right? And it's possible that, for example, Mitt Romney is 50 units north-east from you and Barack Obama is 55 units north-west and Jill Stein is 85 units south-west. You can rank candidates from least far to most far.
- Imagine that every voter for a given candidate does this. All of them. You go talk to all of Gary Johnson's voters. Now, if the map is truly representative, those voters are going to be fairly clustered. And if Johnson is a perfect match, then he's going to be at the exact midpoint intersecting all of those voters. In practice, there's going to be some level of imperfection.
- The candidate who votes are being "stolen" from--again, I do not accept this language--no one deserves votes and no one steals votes--is the candidate who is closest to the mean voter of the third-party candidate. So if you believe that most Gary Johnson voters would have picked Mitt Romney second, on average, you believe Gary Johnson is "stealing" votes from Mitt Romney. If you believe they would have picked Barack Obama second, on average, you believe Gary Johnson is stealing votes from Barack Obama.
- Again, don't use the steal language. A vote for Jill Stein negatively impacts Barack Obama, it benefits Romney, and it is vote splitting. You could even use syphoning. But stealing implies she didn't deserve it or he did and that's not how things work